Sea ice

Iceland is located where the warm water from the Atlantic and the cold water from the Arctic meet. The Irminger current, a branch of the North Atlantic Drift, is deflected westwards by a submarine ridge and flows along the south and west coast before it sinks below the

East Greenland current. A branch of the East Greenland current flows around the north and east coast of Iceland and sometimes brings drifting sea ice close to the coast. On some occasions, ice drifts further west along the east coast of Greenland through the Denmark Strait. In a 'normal' sea-ice year, the edge of the ice is about 90-100 km from the northwest coast of Iceland from January to April. In a mild year, the edge is ca. 200240 km off the coast, in a severe year the ice extends along the northern coast, and in an extreme year the ice may reach the south coast. Koch (1945) reconstructed variations in sea-ice extent since about ad 800 based on a compilation by Thoroddsen. His reconstruction was based on stretches of coast that were 135 km long, and his index was the product of the number of weeks that sea ice was observed and the number of stretches from which sea ice was seen (Fig. 6.6).

A compilation by Bergthorsson (1969) of severe ice years since Norse settlement shows common features with Koch's reconstruction. Due to a strong correlation between air temperature and sea ice, Bergthorsson (1969) reconstructed decadal running means of annual mean temperature variations from the sea-ice data (Fig. 6.7). Sigtryggsson (1972) prepared a more detailed reconstruction of annual variability of sea-ice extent. Based on a critical examination of available data, Ogilvie (1984) made a new reconstruction of sea-ice extent from the Medieval period to ad 1780.

From the documentary climate data available from Iceland, Ogilvie (1998) suggested that colder conditions prevailed during the last decades of the twelfth century, and that there was much sea ice in the seas between Iceland and Greenland around the mid to latter part of that century. The fourteenth century seems to have been very variable, however, with cold periods around 1320 and in the late 1340s. From the 1350s to the end of the 1370s, the climate was generally cold with famine. The sailing route from Iceland to Greenland was changed in the 1350s due to sea ice. Icelandic sources also indicate extensive sea ice some time around the mid to late 1300s. Cold

CO o

Year AD

Figure 6.6 Sea-ice incidence around Iceland (Koch, 1945) from the ninth to the twentieth century.

DECADAL RUNNING MEANS OF TEMPERATURE

DECADAL RUNNING MEANS OF TEMPERATURE

ICE INCIDENCE
Figure 6.7 Decadal running means of annual mean temperature extended back from ad 1950 to 1600 on the basis of sea-ice incidence. (Adapted from Bergthorsson, 1969)

1978 1980 1982 1984 1986 1988 1990 1992 1994 1996

Year

Figure 6.8 Monthly anomalies of Arctic ice extent between November 1978 and August 1995. (Adapted from Bjorgo et al., 1997)

1978 1980 1982 1984 1986 1988 1990 1992 1994 1996

Year

Figure 6.8 Monthly anomalies of Arctic ice extent between November 1978 and August 1995. (Adapted from Bjorgo et al., 1997)

years are mentioned for 1405 and 1422-1426. A model describing sea ice/climate/EL A relationships in northern Iceland has been developed (Stotter et al, 1999), providing a calibrated proxy climate record as a basis for palaeoclimatic reconstruction for the Holocene.

Satellite passive microwave sensors are the most appropriate means of investigating the global sea ice cover (e.g. Bj0rgo et al., 1997). Global sea-ice cover from 1978 to present is provided by the Nimbus 7 Scanning Multichannel Microwave Radiometer (SMMR) and the Defence Meteorological Satellite Program (DMSP) Special Sensor Microwave Imager (SSMI). Statistical analysis on the time series (16.8-year observation period) suggests the decreases in Arctic ice extent and ice area to be 4.5 per cent and 5.7 per cent, respectively (Fig. 6.8). From November 1978 to December 1996, the areal extent of sea ice decreased by 2.9 ± 0.4 per cent per decade in the Arctic, with extreme minima recorded in 1990, 1993 and 1995 (Maslanik et al., 1996). In the Antarctic, however, the area of sea ice extent increased by 1.3 ± 0.2 per cent per decade (Cavalieri et al, 1997). The hemispheric asymmetry is consistent with the modelled response of climate warming due to atmospheric increase in C02. Situations with minimum ice cover along the Siberian coast are characterized by warm, windy conditions in May and continued warm weather in June, followed by strong coastal winds in August leading to final break-up and retreat of the pack ice (Serreze et al, 1995).

The National Snow and Ice Data Center (NSIDC) in Boulder, Colorado, USA, currently archives and distributes two sea-ice data-sets produced using the NASA Team Algorithm applied to satellite passive microwave data. These are the DMSPSSM/I Daily and Monthly Polar Gridded Sea Ice Concentrations, and Sea Ice Concentrations from Nimbus-7 SMMR and DMSPSSM/I Passive Microwave data. Both data-sets are derived from 25 x 25 km gridded brightness temperatures in polar stereographic projection, and provide users with daily- and monthly-averaged sea-ice concentrations for both hemispheres. The NSIDC has also processed daily and monthly total sea-ice extents and total ice-covered areas. These data are provided to investigate interannual variability and trends in sea-ice cover. Total sea-ice area and extent in the northern and southern hemispheres are computed for each day with data. Anomalies are computed from the monthly mean values for November 1978 to November 1987, and February 1988 to December 1996.

Satellite images from the NSIDC World Data Center-A for Glaciology show that between 15 February 1998 and 18 March 1999, the Larsen B ice shelf in Antarctica lost approximately 1839 km2.

0 0

Post a comment